Lithographic imaging systems are high precision, high cost optical systems. As the critical dimensions of the lithographic systems are decreasing, the imaging systems are subject to pressure to improve the accuracy of the images they form. Some lithographic imaging systems employ image correction to reduce errors in images they form. However, lithographic imaging systems with image correction are complex and difficult to align and to maintain in alignment. These complex systems are often subject to misalignment due to environmental thermal changes so that the imaging equipment must be maintained in a thermally stable environment.
FIG. 1 is a cross-sectional schematic side view of an Offner imaging system used in present day lithographic systems. The Offner imaging system 10 is a concentric imaging system having a primary mirror 12 and a secondary mirror 14. The primary mirror 12 has a concave spherical surface 13. The secondary mirror 14 has a convex spherical surface 15. The radius of curvature of the convex spherical surface 15 is about half the radius of curvature of the concave spherical surface 13. The convex spherical surface 15 and the concave spherical surface 13 have centers of curvatures positioned at about the same point 17 indicated by an X on the optical axis 16 shared by the primary mirror 12 and the secondary mirror 14. The Offner imaging system 10 has an object plane 26 and an image plane 27.
In operation, an optical beam 21 propagating from object 18 located at the object plane 26 is directed towards primary mirror 12. Object 18 may be a spatial light modulator or other photolithographic reticle. The optical beam 21 is sequentially reflected by the concave spherical surface 13, the convex spherical surface 15 and the concave spherical surface 13. The second reflection by the concave spherical surface 13 directs the optical beam 21 out of the Offner imaging system 10. The Offner imaging system 10 forms a real inverted image 19 of an object 18 at an image plane 27 spatially removed from the object plane 26. A spatial light modulator located in the object plane 26 is imaged in a one-to-one dimensional relationship on a workpiece. The workpiece may be, for example, a wafer located at the image plane 27.
Reflection of the optical beam 21 by concave spherical surface 13 and convex spherical surface 15 produces no chromatic aberration. If the radius of curvature of the secondary mirror 14 is half that of the primary mirror 12, all 3rd order Seidel aberrations such as spherical, astigmatism, coma, field curvature and distortion are zero in the image plane 27. However, higher order astigmatism is problematic. Increasing the radius of curvature of the secondary mirror 15 from the 2:1 ratio with the radius of curvature of primary mirror 12 introduces some 3rd order astigmatism that cancels with the higher order astigmatism in a narrow annular region of the image plane 27. Any image formed within this annular region is well corrected. Unfortunately, the well-corrected region is a relatively small region of the image plane 27.
Some digital photolithography systems use a reticle that is dynamic, not fixed. In such systems, light is reflected at, transmitted through or emitted from a spatial light modulator located in the object plane 26. The spatial light modulator has a high aspect ratio. The aspect ratio of the spatial light modulator is the ratio of the length to the width of the spatial light modulator. A high aspect ratio is a ratio of more than 5:1. A typical spatial light modulator for a digital lithography system has dimensions of 75 mm to 1 mm for a 75:1 aspect ratio. To fit the complete image of the spatial light modulator within the well-corrected annular region of the image plane 27, requires that the diameter of the primary mirror 12 be large. For a 1 mm by 75 mm reticle image to be within the well-corrected annular region of the primary mirror 12, the diameter of the primary mirror would be about 651 mm. A concave mirror of this size is very expensive.
What is needed is a way to reduce the diameter of the primary mirror 12 in an optical imaging system for digital lithography capable of imaging a high aspect ratio reticle within the well-corrected region. This would reduce the cost of the primary mirror. Specifically, it is desirable to reduce the diameter of the primary mirror 12 to less than 200 mm, since fabrication costs increase rapidly for mirror diameters greater than 200 mm.